This invention relates to a method for effectively coating inorganic particulate solids with polymers. More particularly, this invention relates to a method for effectively coating the surface of inorganic particulate solids with a polymer which comprises polymerizing a vinyl monomer by using as polymerization initiator system a combination of inorganic particulate solids per se and bisulfite ions.
A method is known for coating particulate solids with polymers which comprises dispersing particulate solids into water or an organic solvent, adding thereto a vinyl monomer, and effecting polymerization of the monomer in the presence of a water-soluble or oil-soluble polymerization initiator, thereby forming a polymer on the surface of the particulate solids. However, glass surfaces have poor affinity for polymers and display insufficient binding action for polymers. Accordingly, it is extremely difficult to manufacture glass products having an even and sufficiently thick polymer coating by this known method.
On the other hand, there are known as ways for manufacturing artificial abrasive material (1) a process wherein abrasive particles are mixed with methyl methacrylate and a polymer thereof and the mixture is heated and compressed in the presence of a peroxide (Japanese Patent Publn. No. 1667/Sho. 24), (2) a process wherein abrasive particles and plastic powder are kneaded together with an unvulcanized rubber composition and then the kneaded mixture is molded into a desired shape and cured by heating (Japanese Patent Publn. No. 1897/Sho. 29) and (3) a process wherein a mixture of abrasive particles and polymer powder is incorporated with a monomer capable of dissolving or swelling the polymer and thereafter the mixture is heated and compressed (Japanese Patent Publn. No. 9547/Sho. 35). These known processes utilize adhesive polymers for binding abrasive particles. However, these processes have some drawbacks: the binding force between the abrasive particles and polymer is small, the strength of abrasive mass is low, and local agglomeration of abrasive particles in the polymer binder tends to take place so that products of a given uniform quality cannot be obtained.
For making casting sand, it is known to coat sand particles with a partial reaction product between a phenol resin and hexamethylenetetramine, and to apply to sand particles a surface coating layer of a resin formed by temporarily fusing a powdery resin on the particle surface. In these polymer-coated sand particles, the polymer coating is fixed onto the surface of the particles mainly by physical binding force. Accordingly, such coating tends to peel off during transport or storage. Moreover, the production of the coated sand particles necessitates heating and cooling operations so that the cost of equipment increases and the required operations more complicated and troublesome. On the other hand, the use of a relatively large amount of a polymer is required for completely binding individual sand particles and, if dies are manufactured from casting sand containing such a large amount of a polymer, they will inevitably evolve gases and cause deformation or destruction when molten metal or the like is cast therein.
The present inventors already found that when a transition metal or a compound thereof was brought into contact with a vinyl monomer in the presence of bisulfite ions, the combination of the transition metal, or a compound thereof, and the bisulfite ions functioned as a polymerization initiating system and a polymer was preferentially produced on the surface of particles of the transition metal or a compound thereof thereby forming particles evenly coated with the polymer. As the result of further research, it has now been found that equivalently good results can also be obtained when glass particles, abrasive particles and sand particles are used in lieu of transition metal particles.